Evaporation of metallic salts



Nov. 10, 1942.

G. B. SABIN E EVAPORATION OF METALLIC SALTS Filed Sept. 28, 1940 GEEIREE BEABINE IN V EN TOR BY W/QW ATTORNEY Patented Nov. 10, 1942 EVAPORATION OF METALLIC SALTS George Burr Sabine, Rochester, N. Y., assignor to Eastman Kodak Company; Rochester, N. Y., a corporation of New Jersey Application September 28, 1940, SerialNo. 358,854

4 Claims.

This invention relates to a method of coatin materials for evaporation and specifically to the coating of glass with metallic fluorides.

Attention is directed to copending applications 358,855, now U; S. Patent 2,260,470 McLeod, 358,850, now U. S. Patent 2,271,239 Vokes, 358,- 845, now U. S. Patent 2,286,819 Lee, and 358,846,

now U. S. Patent 2,263,008 McRae, all filed concurrently herewith. The present invention relates to and can be used with the apparatus described in those applications.

In a process for the coating of salts by evaporation, it is customary to form a head of the material supported in a filament and. then to heat this head in a vacuum so that particles of the material are projected from the bead and deposit on any surface in unobstructed view thereof. It is an object of the present invention to provide a method of forming the bead for evaporation.

It is an object of the invention to provide a method-of preparing such a head which method will be free of crepitation or violent jumping about which may cause the salt to escape from the filament before the head is ready.

It is also an object of the invention to provide a method of forming a bead in a vacuum which method will not interfere with the vacuum.

It is well known that satisfactory evaporation may be secured when the material to be evaporated is in the form of a hot bead or heated globule which is supported in a coiled filament as mentioned above, the surface tension of the globule being sufiicient to prevent it from falling out between the coils of the filament. Usually the head is formed and allowed to cool, at which time it takes up a position in the bottom of the coiled filament bulging between the coils thereof. Many methods of forming such a bead are known and are now in use. For example, a large natural crystal of the salt, e. g. magnesium fluoride, may by placed in the filament and heated, by passing current through the filament, so that th crystal melts and forms the type of head just described. However, natural crystals sometimes contain closed air chambers and all of them include either water pockets or water of crystallization. Either of these conditions have two very undesirable efiects, namely, the escaping of the air or water vapor interfere with the formation of a vacuum and requires that the pumping operations be resumed after the bead is formed. The other effect is that the escaping of the air or steam while the crystal is being melted down causes violent decrepitation in. which the material often chambers.

escapes from the filament spoiling a whole run.

A second method which has previously been proposed is to place the material in powdered form in a small molybdenum dish in the filament. However, the shadowing effect of the dish causes an unevenness of the coating especially when the surfaces to be coated are placed directly below the dish.

A third method is to mix the powder with water to form a paste which is forced into the filament and then melted down to the required bead. However, as before, the water vapor interferes with the vacuum and causes sputtering. All of these methods require an unnecessary length of time for the preparation of the bead and if the bead jumps out of the filament require that the whole operation be started over again.

According to the present invention a head is formed by melting down in the filament a pellet compacted from the salt in powder form. That is, a pill is made by direct compression of a powder with substantially no binder and this pill is placed in the filament wherein it is heated, melting down to form the required bead. Since the pellet has no binder and no water other than that minute amount present in the original powder, it is porous and substantially free of any closed Actually the pellet is quite hard, but the air can escape easily therefrom when the surrounding atmospher is evacuated. Furthermore, there is no enclose air or water vapor to cause violent jumping about of the pill.

The objects and advantages of this invention and the invention itself will be fully understood from the following description when read in connection with the accompanying drawing in which:

Fig. 1 is an elevation in cross section of apparatus for coating materials by evaporation.

Figures 2, 3, and 4 show filaments containing pills as used in the present invention.

Figure 5 shows a simple but satisfactory method of forming a pill of metallic fiuoride.

In Fig. 1 bell jar l0 supported on a metal taole H is adapted to be evacuated through an outlet I2. In the container thus formed there is a concave dish l3 on which lenses I 4 are placed to be coated by evaporation of a fluoride in a filament I 5. The filament I 5 is supported and recolved current through supports H3 or by any equiv alent arrangement such as described in McRaes application referred to above.

The beads to be used during the evaporation are formed in the filament l5 and may either be prepared in advance or may be prepared immediately before the actual coating operation using the same vacuum for both steps. In order to prevent any coating of the material before the bead is ready, a shield ll is pivotally supported under the filament I5. After the bead has been prepared, a solenoid l8 releases a detent l9 allowing the shield I! to swing out of the way. The bead in the filament I is usually allowed to cool before the shield I1 is removed and then is heated again by passing current through the leads l6 so that evaporation occursin the well known way.

Various forms of filaments are shown greatly enlarged in Figs. 2, 3, and 4. In Fig. 2 a horizontal helical coil carries two cylindrical pills 2| and 22. These pills or pellets consist of a metallic fluoride or other salt compacted from a powder. This filament when placed in the position of filament l5 of Fig. 1 may be heated in a vacuum causing the pills 2| and 22 to melt down to form a bead bulging from the coils of the filament. In practice I have found it satisfactory to make these pills cylindrical about A; of an inch in diameter and about of an inch long. Two or three pills may be placed in a coil filament whose internal diameter is slightly larger than that of the pills and whose total length is about 1 inch. The only reason for using three small pills instead of one large one is for convenience in making the pills and in handling them.

In Fig. 3 the filament 23 is coiled in the form of a cone with the apex down. A pill 24 is supported in this cone. An improved form of this conical arrangement is shown in Fig. 4 wherein the filament 25 was wound on a wood screw rather than a pure cone and thus consists of a cylinder which tapers gradually at the bottom. A pill 26 is supported in the conical part of this filament 25 and the cylindrical portion keeps the pill from falling out.

In Fig. 5 a metal plate 3| having a hole 32 drilled therethrough is placed on a base plate 30. A salt such as a metallic fluoride or mixture of fluorides in powdered form 33 is inserted in the hole 32 and is compacted by pressing with a steel rod 34. I have found it convenient to make the hole-32 by direct drilling and then to reverse the drill in the drill press so that the fiat end thereof can be used for the rod 34. After a hard pellet is formed from the powder 33, the plate 3| is raised and the rod 34 is used to eject the pill.

This pill compacted from the fluoride in powder form is hard, easy to handle and is porous and substantially free from closed chambers so that when the surrounding atmosphere is evacuated it (the pill) will not withhold air or water vapor to be given off during subsequent heating and when it is melted down in the filament I 5 to form of a bead therein, it will not sputter and jump about. The pellets should be made from the powder 33 entirely by pressure and with substantially no binder so that porosity of the pellet is assured.

Having thus described the preferred embodiment of my invention, I wish to point out that it is not limited to these specific arrangements but is of the scope of the appended claims.

What I claim and desire to secure by Letters Patent of the United States is: I

1. In a process for evaporating metallic fluorides in a vacuum, the method of forming a bead of the metallic fluoride in a filament comprising melting down in the filament a pellet compacted from the fluoride in powder form 2. In a process for evaporating metallic fluorides in a vacuum, the method of forming a head of the metallic fluoride in a filament comprising melting down in the filament a hard pellet compacted with substantially no binder from the fluoride in powder form.

3. In a process for evaporating metallic fluorides in a vacuum, the method of forming a head of the metallic fluoride in a filament comprising melting down in the filament a pellet compacted from the fluoride in powder form, said pellet being hard, porous and substantially free of closed chambers.

4. In a process for evaporating salts supported in a vacuum by a filament adapted to be heated, the method of forming a bead of the salt in the filament which comprises melting down in the filament a pellet compacted of the salt only, said pellet being porous and substantially free of closed chambers.

GEORGE B. SABINE. 

